Decreasing soil-frost depth and its influence on the snowmelt infiltration in Tokachi, Hokkaido, Japan

摘要

A dramatic reduction in soil frost depth occurred in the Tokachi region in northern Japan over the last 20 years. Since soil frost strongly affects snowmelt infiltration and runoff, the reduction in frost depth may have altered the water and nutrientcycles in this region. Soil temperature, water content, matric potential, and snowcover were monitored to quantify snowmelt infiltration flux at an agricultural field for five winters that had various soil frost conditions. When snowmelt began, the soilfrost was 0.1 to 0.2 m thick in three winters and was absent in two other winters, providing a unique opportunity to compare snowmelt infiltration under frozen and unfrozen conditions. Most of the snowmelt water infiltrated into the soil under both frozen and unfrozen conditions indicating that the frozen soil layer of less than 0.2 m did not impede infiltration. Previous frost conditions in agricultural fields was simulated by the snow removal manipulation, which induced deep penetration of the freezing front. The snowmelt infiltration was restricted by a thick (about 0.4 m) frozen layer in this field. These results imply that a regional-scale change in the soil water dynamics has probably occurred with the reduction of frost depth in Tokachi.